[linux-lvm] lvm on loopback mount problems

[Montgomery, Kendal L]

I've seen a few things about using lvm on a loopback filesystem, but nothing
concrete, so I tried it myself.  Most everything works okay until I get to
acutally mounting the filesystem.  Here are the steps I did:

[root at klmontg tmp]# dd if=/dev/zero of=fake_disk.img bs=1M count=500
500+0 records in
500+0 records out
[root at klmontg tmp]# losetup /dev/loop0 fake_disk.img
[root at klmontg tmp]# pvcreate /dev/loop0
pvcreate -- physical volume "/dev/loop0" successfully created

[root at klmontg tmp]# vgcreate /dev/vgtest /dev/loop0
vgcreate -- INFO: using default physical extent size 4 MB
vgcreate -- INFO: maximum logical volume size is 255.99 Gigabyte
vgcreate -- doing automatic backup of volume group "vgtest"
vgcreate -- volume group "vgtest" successfully created and activated
 
[root at klmontg tmp]# lvcreate --name lv_one --size 200M /dev/vgtest
lvcreate -- doing automatic backup of "vgtest"
lvcreate -- logical volume "/dev/vgtest/lv_one" successfully created
 
[root at klmontg tmp]# mke2fs /dev/vgtest/lv_one
mke2fs 1.32 (09-Nov-2002)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
51200 inodes, 204800 blocks
10240 blocks (5.00%) reserved for the super user
First data block=1
25 block groups
8192 blocks per group, 8192 fragments per group
2048 inodes per group
Superblock backups stored on blocks:
        8193, 24577, 40961, 57345, 73729
 
Writing inode tables: done
Writing superblocks and filesystem accounting information: done
 
This filesystem will be automatically checked every 26 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.
[root at klmontg tmp]# e2fsck /dev/vgtest/lv_one
e2fsck 1.32 (09-Nov-2002)
Couldn't find ext2 superblock, trying backup blocks...
e2fsck: Bad magic number in super-block while trying to open
/dev/vgtest/lv_one
 
The superblock could not be read or does not describe a correct ext2
filesystem.  If the device is valid and it really contains an ext2
filesystem (and not swap or ufs or something else), then the superblock
is corrupt, and you might try running e2fsck with an alternate superblock:
    e2fsck -b 8193 <device>
 
[root at klmontg tmp]# e2fsck -b 8193 /dev/vgtest/lv_one
e2fsck 1.32 (09-Nov-2002)
e2fsck: Bad magic number in super-block while trying to open
/dev/vgtest/lv_one
 
The superblock could not be read or does not describe a correct ext2
filesystem.  If the device is valid and it really contains an ext2
filesystem (and not swap or ufs or something else), then the superblock
is corrupt, and you might try running e2fsck with an alternate superblock:
    e2fsck -b 8193 <device>
 
[root at klmontg tmp]# mount /dev/vgtest/lv_one /mnt
mount: you must specify the filesystem type
[root at klmontg tmp]# mount -t ext2 /dev/vgtest/lv_one /mnt
mount: wrong fs type, bad option, bad superblock on /dev/vgtest/lv_one,
       or too many mounted file systems

As you can see, one the filesystem was made, nothing else works. I can't
fsck the filesystem on the new logical volume, or mount it.  This is
obviously a problem since, well, it's useless unless I can mount it.

I'm using RedHat 9 with the  2.4.20-20.9 kernel.  I've also tried this on
RedHat 8 on another box.  I got the latest version of util-linux which is
2.11z and compiled that (in case it was a possible bug in mount 2.11y which
comes with redhat).  I'm using lvm 1.0.3-12 that comes with redhat 9.  I
have not tried getting the latest lvm tools.  That's my next step...

Anyone else had this problem, or tried doing this?  The reason I wanted to
do this in the first place is so I can test snapshots.  I want to be able to
have a filesystem which is static, that I will then take a snapshot of, then
change things on it, and then use the snapshot to bring it back to it's
original form.  I know there is some work on union filesystems or
translucent filesystems, and such things, but none of it seems mature
enough, so this solution seemed like it might work until I hit this snag.

Is anyone else doing this (lvm on loopback)?  Does anyone have it working
correctly?  Am I doing something wrong?  Do I need a different version of
something?

Thanks in advance for any help.

Kendal.